Optical lithography is currently the process of choice for the manufacture of microelectronic circuits (i.e., ICs). Lithography is used in IC manufacture to define the placement, shape and size of various features, such as gates, sources and drains, of field effect transistors, interconnects, trench capacitors and isolation trenches. The basic apparatus includes a source of radiation (typically a mercury lamp), a mask having the desired features, and a lens system for focusing the image of the mask onto an emulsion-coated chip.
Even with optimal optics and process control instrumentation, the minimum feature size obtainable with current lithography technology is limited to about 500 nm using near-UV radiation (365 nm). With the use of shorter wavelengths, such as UV radiation having wavelengths in the order of 250 nm and, again with optimal optics and process control methods, useful feature sizes as small as 350 nm may be obtainable.
Feature sizes even smaller than those described above are desirable for several reasons. If the minimum feature size is reduced to 100 nm, then, for a given size chip, the density of such features are increased in the order of 25 times greater than that obtainable with current technology. Signal processing speed is also increased because the transmission time of signals between, for instance, gates is reduced. The cost per chip decreases with increased density.
In order to produce smaller feature sizes electron beams or ion beams can be used to write patterns. An x-ray lithographic apparatus utilizing an electron beam is disclosed in the U.S. Pat. No. 3,743,042 to Smith et al. However, the writing speeds of such devices are too slow (and the cost is too high) for high volume manufacturing. For higher throughout synchrotrons, free electron lasers and laser produced x-rays are being considered. Though an x-ray source based on a synchrotron is feasible today from a technology standpoint, the cost is too expensive for cost effective manufacture of ICs. Research is being done to try to reduce the cost of synchrotrons that have relevant characteristics. However, even if this desired cost reduction is achieved, system reliability of this type of system will be an overriding issue. Free electron lasers may provide a route to sub 300 nm lithography Free electron lasers may provide a route to sub 300 nm lithography for high volume IC manufacturing, but a breakthrough in technology is needed to realize it, and cost and reliability will be even more of an issue for this technology than it is for synchrotron based systems.
To achieve the desired reduction in feature size, x-ray lithography based on laser produced x-rays is also being investigated and used on a limited basis. U.S. Pat. No. 4,184,078 to Nagel et al. discloses the use of a focused laser beam to produce a high temperature plasma which emits x-rays.
U.S. Pat. No. 4,700,371 to Forsyth et al. improves on Nagel et al. by showing that greater utility can be made of the laser target material by allowing multiple pulses from the laser to impinge on the same target area. They also teach that an enhancement in x-ray intensity can be obtained due to increased directionality of the x-ray radiation. Forsyth et al. require the existence of a cavity in the target material.
G. M. Davis, et al., Applied Physics Letters 53(17), p 1583 (1988), M. Chaker, et al., J. Applied Physics 63(3) p 892 (1988) and E. A. Crawford, et al., J. Vacuum Science Technology B5(6), p 1575 (1987) have demonstrated that point sources of x-rays radiating around 1 keV can be produced with optical-to-x-ray conversion efficiencies in excess of several percent in laser generated plasmas when the input laser intensity is greater than, about, 10.sup.13 W/cm.sup.2. These results were obtained using laser wavelengths between 250 and 1060 nm and with laser pulse widths of a few nanoseconds or less.
FIG. 1 illustrates, in schematic form, the basic apparatus used in x-ray lithography. The apparatus includes a vacuum chamber 13 having a base pressure less than 10.sup.-3 Torr, a laser 15, focusing optics 17, a window 19 through which beam 20 enters chamber 13, a rod 21 (rotated by apparatus not shown) and apparatus, also not shown, for supporting and positioning a wafer 25 and a mask 29. Rod 21 may be of copper or other suitable metal or material. Wafer 25 has an x-ray resist 27 coated thereon. Mask 29 is positioned either in contact with resist 27 or in close proximity, to limit Fresnel diffraction and geometrical distortion. Rod 21 is rotated because each laser pulse vaporizes some material, which eventually will reduce the conversion efficiency. Instead of a rotating rod other devices, such as a moving foil, may be used.
The object of the apparatus of FIG. 1 is to generate, as nearly as possible, a point source of x-rays, of sufficient intensity and sufficient duration to properly expose resist 27. A typical resist exposure requirement would be 100 mJ/cm.sup.2. In operation, a pulse of laser energy having a suitable spot size strikes rod 21, vaporizes a thin layer of material) heats and ionizes the vapor, to produce the x-ray generating plasma 30. Using subnanosecond duration pulses from Nd-glass or Nd:YAG lasers and intensities on target in the order of 10.sup.13 W/cm.sup.2, x-rays in a narrow energy spectrum of 0.75-2.0 keV have been generated. This is the desired spectrum as it minimizes the combined broadening of images due to Fresnel diffraction and photo-electron range in the resist. The conversion efficiency from laser energy to x-ray energy in the desired wavelength band has been measured to be from 1% to 10%. For commercial systems high conversion efficiency is desired because the higher the efficiency the lower the system cost and the higher the expected system reliability.
While Nd-glass lasers are capable of high-energy, short duration pulses, they are limited to low repetition rates because thermal loading of the Nd-glass degrades the ability to focus the laser energy to the required small spot size on target. Also, excessive thermal loading will cause the laser glass to fracture. Reliability and costs of such Nd-glass lasers are also of concern, particularly for high volume manufacturing. At this time through-put (wafers/hour) of the only commercial system presently known to be available (manufactured by Hampshire Instruments, Inc., Marlborough, Massachusetts) is too small relative to its cost to find wide applications for high volume production.
Gas lasers, such as the KrF, XeCl, ArF, sometimes referred to as excimer lasers, and CO.sub.2 lasers, are capable of multi-Joule output pulses at high repetition rates. They are also less expensive and more reliable than glass lasers having the same average power. Gas lasers can have more average power than glass lasers. Also, as the laser medium is gaseous, these lasers, in contrast to glass lasers, cannot be damaged catastrophically by optical feedback. Unfortunately, many of these laser systems store energy in their respective upper laser levels for times short compared to the time during which the upper laser level can be efficiently pumped. As a result, these short storage time lasers produce pulse durations that are determined by pump characteristics. Further, the long pulse widths associated with such gas lasers (greater than 20 nsec) make them unsuitable for the efficient production of the desired x-ray wavelengths. It is not practical to reduce these long pulse widths because of fundamental constraints on the limits of the electrical system used to power the discharge that excites the laser medium.
Since intensities on the order of 10.sup.13 W/cm.sup.2 are required to generate x-rays efficiently, a focal spot size from a one-Joule-per-pulse gas or other short storage time laser of less than 25 .mu.m would generally be necessary. This small spot size combined with the long pulse width results in plasmas with properties that are deleterious to efficient x-ray production. Indeed, G. M. Davis, et al , Applied Physics Letters 53(17), p 1583 (1988), have reported that x-rays in the energy range desired are produced only during the first few nanoseconds following gas laser irradiation; the remaining energy in the laser pulse being essentially wasted. This is illustrated in FIG. 2. Consequently, the poor x-ray production efficiency observed with gas lasers has essentially excluded-these lasers from use in proximity x-ray photolithography.
In articles by T. D. Raymond (one of the co-inventors), C. Reiser, R. G. Adams, R. B. Michie and C. Woods, proceedings of the SPIE 912, p 67 (1988), hereinafter Raymond I, and T. D. Raymond, C. Reiser, R. G. Adams, R. B. Michie and C. Woods, proceedings of the SPIE 912, p 122 (1988), hereinafter Raymond II, the authors disclose a laser system by which energy efficient intensity enhancement can be obtained from gas lasers. The laser system consists of a dye-laser oscillator and a two-stage KrF laser amplifier to provide pulses of laser energy with pulse widths adjustable from 0.8 to 5 nsec, or trains of sub-pulses with interpulse timing adjustable from 2 to 5 nsec. The apparatus disclosed in these two articles was not used to generate x-rays.
While it is known, with the apparatus disclosed in Raymond I and II, to provide single pulses with adjustable pulse widths and trains of sub-pulses, no one has successfully used gas lasers in the efficient production of x-rays. Consequently, the poor x-ray production efficiency previously observed with gas lasers has essentially excluded these lasers from use in proximity x-ray photolithography. Applicants have solved this problem by recognizing that not only are high-intensity/short-duration pulses necessary for the efficient production of x-rays but the importance of spacing between such pulses.
In a paper published subsequent to applicants' invention, E. O'Neill et al., Applied Physics Letters 55(25) p 2603 (1989), disclose an improvement in x-ray production using a train of 100-psec pulses from a XeC laser. In the experiment described in this paper neither the pulse width nor the interpulse timing could be varied O'Neill et al. do not disclose: (1) that the conversion efficiency is strongly dependent on both pulse width and interpulse spacing; (2) that the optimum interpulse time depends on the time required for the election density to dissipate from the vicinity of the target; or (3) a model for plasma expansion that yields an expression for the minimum interpulse time versus focal spot size for optimum x-ray generation.
It is an object of the invention to use pulse train extraction of available laser energy from short storage time lasers (such as an excimer or gas laser) to both efficiently produce laser pulses and to efficiently convert the laser energy to x-ray energy.
It is another object of the invention to use short storage time laser initiated x-ray production in approximately the 0.75- 2.0 keV range, and preferably 0.80-1.3 keV range, by utilizing a train of short duration sub-pulses.
It is also another object of the invention to improve the conversion efficiency of laser energy to x-ray energy from short storage time lasers, including KrF, XeCl, ArF and CO.sub.2 lasers.